First of all, we're calling them Seventeen and Android. Even Japanese fans have taken to this, as you can technically read the Japanese as giving them those names - "Seventeen's a somewhat sickly sort," "Android, with spinning springs," etc.
Anyway, Seventeen and Android aren't in the best of relationships. Android wants her love to make him feel less awful, but Seventeen largely ignores him (or worse - whack!), so he considers himself a "ghost."
This relationship could be somewhat of a metaphor for Hachi's relationship with Vocaloids - he doesn't "show them" his real feelings in his songs and just uses them as tools. But I wouldn't read TOO much into it.
Regardless of the state of their relationship, at any rate, they seem to stick together. Their "ghost ship" moves into a peaceful town, and they decide to stir things up.
They go through town shouting to the people, encouraging them to rise up - cause destruction, make the news, and tell others to do the same, because "it's the truth, so believe it." More and more "ghosts" join them.
By the time they're done, everyone's "with a megaphone," speaking their mind (which they apparently haven't done for 3000 years, so they have plenty of grudges), and it's escalated into a riot.
But has doing this helped Seventeen and Android's relationship any? Nope. He tried to imagine that what they were doing brought them closer - that it'd be "good luck" for their relationship.
However, he knew all along that this was "more or less a lie," so he just wanted to "make the most of it while it lasts."